On Mine the "Android system process" starts around 50-60MB and grows between 5-10 every day.
After reading about a million of random discussions I finally found and tried these bootloader fix instructions:
[MOD][JB][I8160+P] 624 MB accessible RAM on … | Samsung Galaxy Ace S5830, S5830i, Ace II | XDA Forum
and hooray! My available RAM increased from 482MB to 624MB, which makes the experience MUCH better. I STILL have the Android system process leaking memory exactly like before but it seems that thanks to the increased memory I don't have to reboot the phone as often as before.
I am still tempted by bringing the phone to a service centre, see if they can perform a "full" factory reset and fix the issues for good?